High-Sensitivity Nitric Oxide Detection via LIF: Air Quality and Climate Applications

Nitrogen oxides (NOx = NO + NO2) are central to the catalytic production of ozone (O3) and the secondary formation of the hydroxyl (OH) radical. A critical objective for the field of atmospheric chemistry is to understand the implications of a change in anthropogenic NOx emissions for oxidation chemistry. However, this is hindered due to poor understanding of low-NOx environments and a lack of data to constrain these uncertainties. This thesis aimed to reduce these uncertainties by developing a high sensitivity laser-induced fluorescence (LIF) system for nitric oxide (NO) detection. The LIF-NO described achieved a sensitivity comparable to previous work of 13 counts s−1 mW−1 pptv−1, an accuracy of 5.1% + 3 pptv and a detection limit of 9 pptv (SNR = 3) for a 10 Hz integration time or 0.3 pptv at the Allan minimum. A novel reference normalisation procedure was implemented to account for variations in sensitivity resulting from changes in fluorescence quenching rates. The reference normalisation procedure was validated against an Air Quality Design (AQD) chemiluminescence detector for the eddy covariance (EC) flux method at an urban background site. The LIF-NO was equipped with a nitrogen dioxide (NO2) photolytic converter (PLC) and agreed with a NO2 absorption spectrometer for sub-ppbv NOx conditions at a remote marine boundary layer (MBL) site. A photostationary steady-state (PSS) analysis confirmed that the oxidation chemistry of this environment could be explained without invoking additional oxidants. Reactive nitrogen emissions from soils were also investigated using the LIF-NO and a dynamic flux chamber, with fluxes for NO in the range of 0.75 -1.75 nmol[NO] m−2 s−1, and no detectable fluxes of NO2.